1. Field of the Invention
This invention relates to wireless communications systems, and more particularly to cellular communication systems.
2. Description of Related Art
In cellular communication systems, geographic areas or regions are typically divided into cells that are nominally hexagonally shaped. Each cell is allocated one or more radio frequency channels. In a frequency division multiple access (FDMA) system, adjacent or nearby cells are assigned separate frequencies. (The techniques described herein are primarily intended for use in FDMA systems, but may be used in time division multiple access (TDMA) or code division multiple access (CDMA) systems as well). After all available frequencies have been allocated, it is necessary to begin reusing the frequencies. For example, if seven frequencies are available, it is necessary to begin using the first frequency again starting in the eighth cell.
FIG. 1 is a block diagram of a prior art cell configuration showing a problem of frequency reuse. Clusters of seven cells (modeled as hexagons for ease of understanding) form frequency reuse groups or cell groups 1, indicated by bold lines. Seven frequencies are used within each cell group 1, and then reused in adjacent cell groups 1. Within each cell group 1, the pattern of frequency distribution is normally the same. Thus, the center cell 2a of the central cell group 1 a shown uses the same frequency as the center cell 2b of the adjacent cell group 1b.
Because frequencies are reused, two cells operating on the same frequency, though separated geographically, may interfere with each other. This is known as "co-channel interference". The effect of co-channel interference varies with terrain and distance. In cases where path loss conditions favor the desired signal, the co-channel interference may not be strong enough to have a significant impact on receiver performance. In other cases, path loss conditions may cause the difference between the desired carrier power and the interference (known as the "C/I" ratio) to be insufficient for good receiver performance. In many systems this occurs when the C/I ratio is below about 16-17 dB (generally indicative of significant co-channel interference), although the acceptable C/I ratio may be more or less, depending on the nature of the signal and the channel. The overall effect is to create areas within a cell where no good coverage is possible. In a case of seven total frequencies, these bad locations may comprise 40% or more of a typical cell.
The traditional way to mitigate co-channel interference in FDMA systems is to allocate a larger number of frequencies to the service and to devise sparse reuse patterns. A common allocation is a reuse factor of 21 (7 cells with three 120.degree. sectors per cell). However, this method cannot be used when only a small number of frequencies, such as seven, are available.
Recently, the Federal Communications Committee auctioned licenses for frequency spectrum intended for providing improved personal communications services such as two-way paging. These systems are commonly referred to as narrowband Personal Communications Systems, or NPCS. NPCS creates the opportunity for development of a new class of messaging services and instrumentality having improved efficiency and greater functionality in comparison with prior art messaging or paging systems. Two classes of licenses for NPCS are presently available. A first class, referred to hereinafter as "symmetrical" NPCS, comprises two 50 kHz frequency segments. One 50 kHz segment is allocated for communication from a base station to a subscriber device. This frequency segment is commonly referred to as a "forward" or "down" link. Another 50 kHz segment is allocated for communication from the subscriber unit to the base station. This frequency segment is commonly referred to as a "reverse" or "up" link. This class of license is "symmetrical" in the sense that there is equal bandwidth on both the forward and reverse links. Typically, a base station is assigned a forward frequency channel and a corresponding reverse frequency channel for communication with a plurality of mobile units.
A second class of license, referred to hereinafter as "asymmetrical" NPCS, comprises a 50 kHz frequency segment on the forward link and a 12.5 kHz segment on the reverse link. This class of license is "asymmetrical" in the sense that there is less bandwidth available on the reverse link than on the forward link. Typically a base station is assigned a forward frequency channel for communication to a plurality of mobile units. All base stations are assigned the identical reverse frequency channels for receiving signals from the plurality of mobile units. Licensees can and will in some instances acquire contiguous licenses and thereby obtain larger blocks of frequency spectrum and greater bandwidth.
Paging systems typically communicated using only the forward link prior to the development of NPCS. There was very limited communication using the reverse link. The prior art paging systems used "simulcasting" or "broadcasting" techniques to send messages on the forward link. The transmissions of all of the base stations in a service area are identical and are synchronized in a simulcast system. Thus, transmissions from each transmitter station tend to reinforce each other. In many topographical scenarios transmissions received from several simulcast transmitters generally have improved signal strengths when compared to transmissions received from a single transmitter. Consequently, simulcast systems can transmit stronger signals into areas that suffer from large path losses. Disadvantageously, the net data transmission rate supported by simulcast systems is quite low. For example, the total bit rate supportable for an entire metropolitan area is typically only 2,400 bits per second (bps). To date, these low bit rates have been adequate for supporting numeric paging messages in which messages primarily comprise a simple telephone number. However, new services such as alphanumeric paging (where messages comprise a large number of text characters) require increased transmission bandwidth. Consequently, many paging systems are approaching capacity and cannot support such services.
Accordingly, it is desirable to provide a communication system which uses a small number of frequencies while substantially reducing significant co-channel interference. Additionally, it is desirable to enhance the efficiency and to increase the data bandwidth on the reverse link of the cellular communication system. The present invention provides such a cellular communication system.